The invention relates to a filter arrangement with a carrier substrate, a resonator unit, and a reflection element between the carrier substrate and the resonator unit. The invention also relates to an electronic component with a carrier substrate, a resonator unit, and a reflection element between the carrier substrate and the resonator unit, as well to as a method of manufacturing such a component.
The extremely fast developments in the field of mobile telephony and the continuous miniaturization of cordless telephone equipment lead to increased requirements imposed on the individual components. Thus a high selectivity in the high-frequency range is necessary for protecting the receiver from the increasing number of potential interfering signals from other systems.
At the moment, filters with ceramic electromagnetic resonators are used for this purpose in the high-frequency range. A miniaturization of these filters, however, is restricted by the electromagnetic wavelength. Surface acoustic wave (SAW) filters can be constructed so as to be substantially smaller. The reason for this is that the acoustic wavelength is smaller by 4 to 5 orders of magnitude than the electromagnetic wavelength. A disadvantage is, however, that surface acoustic wave filters often have a complicated construction and have to be protected by means of complicated housings.
An alternative is formed by bulk acoustic wave (BAW) filters. Bulk acoustic wave filters have advantages as regards their size, power, and IC compatibility. Bulk acoustic wave filters can be realized with widely varying types of bulk acoustic wave resonators. Thus, for example, single-crystal resonators, resonators with membranes, or resonators with air gaps may be used.
In principle, bulk acoustic wave resonators are built up from three components. The first component generates the acoustic wave and comprises a piezoelectric layer. Two electrodes, which are arranged above and below the piezoelectric layer, represent the second component. The third component has the task of acoustically insulating the carrier substrate from the oscillations generated by the piezoelectric layer.
A possibility for acoustic insulation is formed by reflection layers which are provided between the carrier substrate and the resonator unit. Such reflection layers consist of layers having alternately a high and a low acoustic impedance. The thickness of an individual layer is one fourth of the resonance wavelength. These layers have the purpose of reflecting the acoustic waves at the respective boundary surfaces as well as possible, and thus keeping the acoustic energy inside the resonator unit. The number of layers required depends on the one hand on the differences in acoustic impedance between the individual layers and on the other hand on the resonator quality Q to be achieved.
The individual reflection layers may be made from a wide variety of materials. A material of low acoustic impedance which is often used is SiO2 with an acoustic impedance of 13 Ggmxe2x88x922sxe2x88x921. AlN with an acoustic impedance of 34 Ggmxe2x88x922sxe2x88x921, W with an acoustic impedance of 101 Ggmxe2x88x922sxe2x88x921, and HfO2 with an acoustic impedance of 40 Ggmxe2x88x922sxe2x88x921 are used as materials in layers having a high acoustic impedance.
A bulk acoustic wave resonator is known from WO 98/16957 in which polymers such as, for example, polyimide are used as materials of low acoustic impedance in the reflection layers. The number of reflection layers necessary for acoustic insulation can be reduced owing to a low acoustic impedance of 2 Ggmxe2x88x922sxe2x88x921.
The invention has for its object to provide an improved bulk acoustic wave filter.
This object is achieved with a filter arrangement with a carrier substrate, a resonator unit, and a reflection element between the carrier substrate and the resonator unit which is characterized in that the reflection element comprises a layer of an acoustically reflecting substance.
Preferably, the acoustically reflecting substance belongs to the group of polymers and porous substances.
Porous or polymer acoustically reflecting substances have low acoustic impedance values and thus render it possible for only one layer to suffice for reflecting the total acoustic energy. The condition that the thickness of the layer should be one fourth of the resonance wavelength can even be dropped in this case. This simplifies the construction and renders the manufacture of the filter arrangement less expensive.
It is particularly preferred that the acoustically reflecting substance is an aerogel, a xerogel, a glass foam, a foam-type adhesive, a foamed synthetic resin, or a synthetic resin of low density.
The advantage of these materials is that they have extremely low impedance values of down to 10xe2x88x922 Ggmxe2x88x922sxe2x88x921, depending on the material used. The acoustic impedance thus lies only two orders of magnitude above the acoustic impedance of air (4xc2x710xe2x88x924 Ggmxe2x88x922sxe2x88x921) in the most favorable case.
It may be preferred that the reflection element comprises layers of alternately high and low acoustic impedance, with an acoustically reflecting substance being used for any layer of low acoustic impedance.
Only very few layers are necessary in the reflection element for reflecting the total acoustic energy owing to the low acoustic impedance values of polymers and porous acoustically reflecting substances.
It is preferred that the material of high impedance comprises HfO2, Mo, Au, Ni, Cu, W, Ti/W/Ti, WxTi1xe2x88x92x (0xe2x89xa6xxe2x89xa61), diamond, Ta2O5, Pt, Ti/Pt, or a synthetic resin of high density.
The material may be provided in thin-film processes such as reactive sputtering or vacuum deposition processes such as, for example, chemical vapor deposition (CVD), physical chemical vapor deposition (PCVD), or spin coating.
It is furthermore preferred in this filter arrangement that the thickness of the layers is one fourth of the resonance wavelength each time.
The reflections of each layer will combine in phase in the resonance wavelength in a sequence of layers of alternately high and low impedance which is combined with layer thicknesses of one fourth of the resonance wavelength.
It may be preferred that a further reflection element is arranged on the resonator unit.
The provision of a further reflection element on the resonator unit on the one hand has the advantage that no acoustic vibrations reach the upper side of the filter arrangement. On the other hand, the reflection elements protect the resonator unit from contamination with dust particles and the like. The provision of reflection elements above and below the resonator unit, which in the most favorable case consist of only one layer, is cheaper than other protection techniques such as, for example, a semi-hermetic envelope.
It is also preferred that a protective layer of an organic and/or inorganic material is provided over the filter arrangement.
The filter arrangement is protected from mechanical loads and corrosion through moisture by the protective layer.
It is preferred that the carrier substrate comprises a ceramic material, a ceramic material with a glass planarizing layer, a glass-ceramic material, a glass material, silicon, GaAs, or sapphire.
A carrier substrate of a ceramic material, a ceramic material with a glass planarizing layer, a glass-ceramic material, or a glass material can be inexpensively manufactured and the process cost for these components can be kept low. If the filter arrangement is to be integrated into an IC, the carrier substrate made of a semiconductor material may possibly be provided with a passivating layer, for example of SiO2 or glass.
It is furthermore preferred that the resonator unit comprises a piezoelectric layer of PbZr0.15Ti0.85O3 doped with 2% lanthanum.
PbZr0.15Ti0.85O3 doped with 2% lanthanum has particularly good piezoelectric properties in the filter arrangement.
It may be preferred that an adhesive layer is arranged between the carrier substrate and the reflection element.
This layer serves to fasten the reflection element on the carrier substrate.
The invention further relates to mobile telephone appliances, transmitters, receivers, and wireless data transmission systems which are fitted with a filter arrangement comprising a carrier substrate, a resonator unit, and a reflection element between the carrier substrate and the resonator unit, wherein the reflection element comprises a layer of an acoustically reflecting substance.
The invention also relates to an electronic component with a carrier substrate, a resonator unit, and a reflection element between the carrier substrate and the resonator unit, wherein the reflection element comprises a layer of an acoustically reflecting substance.
Inexpensive components such as, for example, bulk acoustic wave resonators can be manufactured with the use of polymers and porous acoustically reflecting substances of extremely low impedance as the reflection elements. A reduction in the manufacturing cost is achieved first in that only one layer need be provided for the reflection element and in addition in that this layer need not necessarily have a layer thickness of one fourth of the resonance wavelength.
The invention further relates to a method of manufacturing an electronic component with a carrier substrate, a resonator unit, and a reflection element made from an acoustically reflecting substance, in which method
a resonator unit is provided on a substrate layer,
a reflection element of an acoustically reflecting substance is provided on the resonator unit, and
a carrier substrate is fastened on the reflection element, and the substrate layer is removed.